The use of protective coatings on metal substrates for improved corrosion resistance and paint adhesion is common. Conventional techniques for coating such substrates include techniques that involve pretreating the metal substrate with a phosphate conversion coating and chrome-containing rinses. Typical phosphate conversion coatings operate in the range of about at least 1,000 parts per million (“ppm”) of phosphate, which leads to waste treatment issues. The use of such phosphate and/or chromate-containing compositions, therefore, imparts environmental and health concerns.
As a result, chromate-free and/or phosphate-free pretreatment compositions have been developed. Such compositions are generally based on chemical mixtures that in some way react with the substrate surface and bind to it to form a protective layer. For example, pretreatment compositions based on a group IIIB or IVB metal compound have recently become more prevalent.
When processing ferrous metal substrates through a pretreatment composition based on a group IIIB or IVB metal compound, however, the concentration of ferric (Fe+3) iron in a bath of the pretreatment composition increases over time as more iron based metal is treated. In particular, soluble (Fe+2) iron from the substrate becomes insoluble (Fe+3) through Fe+2 concentration build up, oxidation, and subsequent reaction with oxygen and water. The resulting insoluble rust, i.e., hydrated iron (III) oxide (Fe2O3.nH2O) and/or iron (III) oxide-hydroxide (FeO(OH)), flocculates and the insoluble rust particles resist settling out during the mild agitation present while processing parts. As a result, the insoluble rust particles can adhere to or deposit on the substrate and be carried to subsequent processing steps (particularly when filtration equipment is not available), such as a downstream electrocoat bath that is employed to deposit an organic coating. Such cross-contamination can detrimentally affect the performance of such subsequently electrodeposited coatings.
As a result, it is conventional practice in the industry to periodically dilute the pretreatment bath to reduce soluble iron concentration as a preventative measure and to add a replenisher to the pretreatment bath in order to replenish the bath ingredients and to regain coating ability. In some instances, the pretreatment bath has to be removed from the processing line to perform methods for removing rust therefrom. Alternatively, the pretreatment bath must be discharged every one to two weeks and a fresh bath made up. Each of these practices is costly due the significant product loss, waste treatment, and inconvenience.
As a result, it would be desirable to provide improved methods for treating a ferrous metal substrate and for removing soluble iron that address at least some of the foregoing.